The probability of staking a block is based on how many tokens you have and how many others are securing the network. The block time is about 2 minutes, meaning there will be about 720 blocks per day, so it's just a numbers game. If you have 10,000 tokens, and only 10 million other tokens are participating in the staking process your chances of getting a block are pretty high. We'll see how it goes in September, but when the Test Network launches in June we will have a chance to try it out.